1. Technical Field
The present disclosure relates to the field of underwater acoustic (UWA) communications. More particularly, the present disclosure relates to enhanced multicarrier UWA communications using nonbinary low density parity check (LDPC) codes (e.g., regular or irregular LDPC codes).
2. Background Art
In general, underwater acoustic (UWA) communication (e.g., the sending and/or receiving of acoustic signals underwater) is a difficult and complex process. The unique characteristics of water as a propagation medium typically contributes to the problematic nature of UWA communication. For example, due to factors such as multi-path propagation and time variations of the channel, it is necessary to account for, inter alia, small available bandwidth and strong signal attenuation. Moreover, slow propagation speeds typically associated with acoustic signals may lead to significant Doppler shifts and spreading. Thus, UWA communication systems are often times limited by reverberation and time variability beyond the capability of receiver algorithms.
Multicarrier underwater acoustic communication, in the form of orthogonal frequency division multiplexing (OFDM), can be used to address some of the difficulties associated with UWA communications. See, e.g., M. Chitre, S. H. Ong, and J. Potter, “Performance of coded OFDM in very shallow water channels and snapping shrimp noise,” in Proceedings of MTS/IEEE OCEANS, vol. 2, 2005, pp. 996-1001; P. J. Gendron, “Orthogonal frequency division multiplexing with on-offkeying: Noncoherent performance bounds, receiver design and experimental results,” U.S. Navy Journal of Underwater Acoustics, vol. 56, no. 2, pp. 267-300, April 2006; M. Stojanovic, “Low complexity OFDM detector for underwater channels,” in Proc. of MTS/IEEE OCEANS conference, Boston, Mass., Sep. 18-21, 2006; and B. Li, S. Zhou, M. Stojanovic, and L. Freitag, “Pilot-tone based ZPOFDM demodulation for an underwater acoustic channel,” in Proc. Of MTS/IEEE OCEANS conference, Boston, Mass., Sep. 18-21, 2006. OFDM has typically been used because of its capability to handle high-rate transmissions over long dispersive channels. In general, OFDM divides the available bandwidth into a large number of overlapping subbands, so that the symbol duration is long compared to the multipath spread of the channel. As a result, inter-symbol-interference (ISI) may be neglected in each subband, which reduces the complexity of channel equalization at the receiver.
Some of the research associated with OFDM UWA technologies has been focused on how to make OFDM work in the presence of fast channel variations. Experimental results of researchers in the field have demonstrated that OFDM is feasible and flexible for underwater acoustic channels. See, e.g., B. Li, S. Zhou, M. Stojanovic, L. Freitag, and P. Willett, “Multicarrier communications over underwater acoustic channels with nonuniform Doppler shifts,” IEEE J. Oceanic Eng., vol. 33, no. 2, April 2008; B. Li, J. Huang, S. Zhou, K. Ball, M. Stojanovic, L. Freitag and P. Willett, “MIMO-OFDM for High Rate Underwater Acoustic Communications,” IEEE Journal on Oceanic Engineering, vol. 34, no. 4, pp. 634-644, October 2009; and B. Li, S. Zhou, J. Huang, and P. Willett, “Scalable OFDM design for underwater acoustic communications,” in Proc. of Intl. Conf. on ASSP, Las Vegas, Nev., Mar. 3-Apr. 4, 2008.
However, two main hurdles should be adequately addressed to successfully deploy OFDM in a practical system: 1) Plain (or uncoded) OFDM has poor performance in the presence of channel fading, since it typically does not exploit the frequency diversity inherent in the channel; and 2) OFDM transmission typically has a high peak-to-average-power ratio (PAPR), and thus a large power backoff reduces the power efficiency and limits the transmission range.
Dedicated studies of coding for underwater acoustic communication are limited. Typically, UWA communication systems employ coding schemes known in the art. For example, trellis coded modulation (TCM) has been used together with single carrier transmission and equalization. See, e.g., M. Stojanovic, J. A. Catipovic, and J. G. Proakis, “Phase-coherent digital communications for underwater acoustic channels,” IEEE Journal of Oceanic Engineering, vol. 19, no. 1, pp. 100-111, January 1994. Similarly, convolutional codes and Reed Solomon (RS) codes have also been examined for applications in underwater acoustic communication. See, e.g., A. Goalic, J. Trubuil, and N. Beuzelin, “Channel coding for underwater acoustic communication system,” in Proc. of OCEANS, Boston, Mass., Sep. 18-21, 2006. Further, space time trellis codes and Turbo codes in conjunction with spatial multiplexing have been used for a single-carrier underwater system with multiple transmitters. See, e.g., S. Roy, T. M. Duman, V. McDonald, and J. G. Proakis, “High rate communication for underwater acoustic channels using multiple transmitters and space-time coding: Receiver structures and experimental results,” IEEE Journal of Oceanic Engineering, vol. 32, no. 3, pp. 663-688, July 2007. In regards to the coding of the OFDM signal, serially concatenated convolutional codes have been used and tested with a non-iterative receiver. See, e.g., M. Chitre, S. H. Ong, and J. Potter, “Performance of coded OFDM in very shallow water channels and snapping shrimp noise,” in Proceedings of MTS/IEEE OCEANS, vol. 2, 2005, pp. 996-1001.
Low density parity check (LDPC) codes are known to be capacity-achieving codes. See, e.g., R. G. Gallager, Low Density Parity Check Codes. Cambridge, Mass.: MIT Press, 1963. LDPC codes have been extensively studied for wireless radio systems. Relative to binary LDPC codes, one advantage of nonbinary LDPC codes is that they can be matched very well with underlying modulation. For example, nonbinary LDPC codes were first combined with high order modulation in radio communication systems with two transmitters and two receivers. See. e.g., F. Guo and L. Hanzo, “Low complexity non-binary LDPC and modulation schemes communicating over MIMO channels,” in Proc. of VTC, vol. 2, pp. 1294-1298, Sep. 26-29, 2004. Further, simulations have shown that an iterative receiver with nonbinary LDPC codes over GF(16) can outperform the best optimized binary LDPC code in both performance and complexity, while a non-iterative receiver with regular LDPC cycle code over GF(256) can achieve much better performance with comparable decoding complexity compared to the binary iterative system. See, e.g., R.-H. Peng and R.-R. Chen, “Design of nonbinary LDPC codes over GF(q) for multiple-antenna transmission,” in Proc. of Military Communications conference 2006, Washington, D.C., Oct. 23-25 2006, pp. 1-7.
Current OFDM UWA communication systems fail to adequately address the shortcomings of OFDM technologies. Specifically, uncoded or plain OFDM has poor performance in the presence of channel fading and OFDM transmission has a high peak-to-average-power ratio (PAPR). Due to the limited bandwidth, high order constellations are more desirable for multicarrier underwater communication. These and other inefficiencies and opportunities for improvement are addressed and/or overcome by the apparatus, systems and methods (e.g., LDPC based apparatus, systems and methods) of the present disclosure.